1. Field of the Invention
The present invention relates generally to optical communications, and more specifically to devices and methods providing dispersion compensation of an optical signal.
2. Technical Background
As the bit rates of optical communications systems increase, the deleterious effects of dispersion in the optical fibers used in long-distance transmission become increasingly important. Dispersion causes an optical pulse to spread out in time; the longer wavelength components of the pulse travel along the fiber at a different rate than do the shorter wavelength components of the pulse. Typically, long-distance transmission fibers (e.g. LEAF®, available from Corning Incorporated of Corning, N.Y.) have a small but non-negligible positive dispersion, causing the shorter wavelength components to arrive at a network node before the longer wavelength components. Such a pulse is said to be positively chirped. This temporal spreading can cause loss of signal fidelity and an increase in bit error rate.
Conventional methods of dispersion compensation use dispersion compensating fiber to reverse the effects of dispersion in the transmission fiber. Dispersion compensating fiber typically has a large negative dispersion to counteract the positive dispersion of the transmission fiber. In one type of conventional dispersion compensating device, a dispersion compensating fiber is packaged on a spool in a module. The length and dispersion properties of the dispersion compensating fiber are chosen to balance the dispersion of the span of transmission fiber to which it is coupled. A positively chirped optical signal from the transmission fiber is propagated through the dispersion compensating fiber, and the negative dispersion of the dispersion compensating fiber removes the positive chirp from the optical signal, forming a signal with essentially no chirp. While such conventional methods are relatively simple to implement, they are limited in that they are passive; the dispersion compensation properties of such passive dispersion compensation devices are determined by the length and dispersion properties of the dispersion compensating fiber. If the chirp of the incoming optical signal is substantially different than that for which the device was designed, the device will be ineffective at providing an essentially chirp-free optical signal. Such devices are also generally unable to remove all of the chirp of the optical signal, imposing a residual dispersion on the transmission link. In an optical communications system with large distances of transmission fiber and multiple passive dispersion compensation devices, the residual dispersion can have a significant impact on the quality of the optical signal. Residual dispersion is especially damaging in long-distance (e.g. >1000 km) 10 Gb/s systems as well as in 40 Gb/s systems.
Wavelength division multiplexing (WDM) techniques have become ubiquitous in optical communications. As such, optical signals typically have a plurality of wavelength channels over a relatively broad (e.g. tens of nanometers) range of wavelengths. It is therefore desirable for dispersion compensating devices to provide dispersion compensation over a broad range of wavelengths. Conventional grating-based devices and planar waveguide-based devices provide controllable dispersion compensation only over a relatively narrow band of wavelengths, and are very expensive to produce. Micro-optic-based dispersion compensators have also been proposed. While these devices can provide broadband compensation, they suffer from high excess loss and low reliability.